Lithium ion secondary batteries have high energy density and high capacity, and thus are widely used as driving power sources for mobile data terminals and the like. In recent years, the use of lithium ion secondary batteries for industrial applications, for example, installation in electric and hybrid automobiles that require large capacity is also increasing, and studies for increasing the capacity and performance even more are conducted. One of such studies attempts to increase the charge and discharge capacity using silicon or tin, which can occlude a large amount of lithium per unit volume, or an alloy containing silicon or tin, as an active material for a negative electrode.
However, when an active material having a large charge and discharge capacity, such as silicon, tin, or an alloy containing silicon or tin, for example, is used, the active material significantly changes its volume in accordance with charging and discharging. For this reason, if polyvinylidene fluoride or a rubber resin, which are used for conventional electrodes, is used as a binding agent (binder), the problem arises in that due to breakage of an active material layer or the occurrence of detachment at an interface between a current collector and the active material layer, a current collecting structure of the electrode may be broken, and thus the cycle characteristics of the battery may easily deteriorate.
Thus, there is a demand for a binder for an electrode, the binder being unlikely to cause breakage or detachment of an electrode even when a significant change in volume occurs and having high toughness in a battery environment.
Patent Document 1 discloses that when a polyimide resin is used as a binding agent for a negative electrode of a lithium secondary battery, the battery capacity hardly decreases even after repeated charge-discharge cycles, and thus a long cycle life is achieved. According to this document, the electrode is produced by performing heat treatment at 350° C. for 2 hours (see Examples 1 and 2).
Patent Document 2 discloses a binder resin composition for an electrode, the binder resin composition including a specific polyamic acid and a solvent and exhibiting a low degree of swelling in an electrolyte solution and excellent toughness (high breaking elongation and breaking energy). Moreover, it is disclosed that during production of the electrode, heat treatment at a relatively high temperature is required so that an imidization reaction proceeds to a sufficient extent.
Patent Document 3 discloses a resin composition for an electrode of a lithium ion secondary battery, the resin composition containing a polyimide resin having a carboxyl group and an epoxy resin.
On the other hand, Non-Patent Document 1 states that a lower degree of swelling of a binder resin for an electrode in an electrolyte solution leads to a higher discharge capacity retention after charge-discharge cycles and is thus preferred.